Passive receiver networks are regarded as an accurate and effective technique for the localisation of an RF signal source and/or signal reflecting object. The major advantage of passive receiver networks is that they operate without the need for knowledge of the incoming signal and they do not require transmission of a localisation signal.
The common technique for localizing in passive receiver networks is multilateration, also known as hyperbolic positioning, which is based on the time-difference-of-arrival (TDOA) between the various receivers. As with triangulation, to localise uniquely a single object, a specific number of receivers is required. In passive receiver networks, it is required to maintain a larger number of receivers than objects (for 2-D localisation a minimum of four receivers is required for two or more objects). When this condition is not met, it results in intersections which provide false object positions, without any real substance, which are called “phantoms”. It is therefore imperative to remove these phantoms in order to estimate the true location of the objects.
There have been numerous techniques proposed for this “spatial filtering”, i.e. the removal of the phantoms in the localisation process using passive networks. However, the solutions are based on angular and elevation measurements, exhaustive processing, hardware alterations and tracking. None of these solutions can be applied to a scenario in which:                There are no multiple acquisitions, which means that tracking cannot be used.        The receivers might be stationary; therefore it is impossible to move them in order to identify the phantoms as the intersections that move to a physically impossible position.        The available receivers do not have the capability of measuring anything else apart from time, so that angular and elevation information is not available.        Each receiver operates stand-alone and transmits the received signal and/or other related information from the signal and the position of the receiver, to a data processing unit.        